Detection of Neutrons: Part I Ralf Nolte

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• Introduction

– Neutrons in Science and Technology

– Interaction of Neutrons with Matter

• Neutron Detection

– General Properties of Detectors

– Detectors for Thermal and Slow Neutrons

– Detectors for Fast Neutrons

• Recoil Detectors: Prop. Counters, Scintillation Detectors, Recoil Telescopes

• (Fission) Ionization Chambers

• Techniques for Neutron Measurements

– Time-of-flight

– Spectrometry

– Spatial Neutron Distribution

• Absolute Methods, Quality Assurance

– Associated particle methods

– Key comparisons

Table of Contents

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Where to Find More Reference Material?

• W.D. Allen: Neutron Detection (1960)

• J.B. Marion, J.L. Fowler: Fast Neutron Physics (1960)

• K.H. Beckurtz, K. Wirtz: Neutron Physics (1964)

• W.R. Leo: Techniques for Nuclear and Particle Physics Experiments (2nd ed. 1994)

• G. Knoll: Radiation Detection and Measurement (4th ed. 2010)

Conference proceedings:

• H. Klein et al. : Proc. NEUSPEC 2000 NIMA 475 (2002)

• SORMA, Crete, ND, …

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Historical Prelude: Chadwick‘s Discovery of the Neutron

• Sir James Chadwick (1932)

• Correct explanation of the

experiments by I. Curie and F. Joliot

• All elements of a modern neutron

detector were present:

– Neutron converter

– Detector for charged particles

…the man who never laughed Ref.: J. Chadwick, Nature 132 (1932) 3252

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• Laboratory for fundamental physics: EDM, …

Neutrons in Science …

• Ideal tool for probing matter:

– No Coulomb force: deep penetration

– Strong Interaction: isotope-specific detection

– Magnetic moment: magnetic structures

– Low energies: crystal structures

EdB ~L

UCN detection:

3He(n,p)3H

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… Technology

• Neutrons can be used to produce energy

– Fusion

– Fission

• Biggest disadvantage: the (free) neutron is unstable:

t 880 s

• Intense neutron sources require considerable efforts: – Reactors

– High-power low-energy accelerators

– Spallation sources

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… and the World Around

• Cosmic Neutron:

– Production in the atmosphere by

galactic radiation

– Production on the sun

• Neutron monitors:

Diagnostic for solar processes

• Radiation protection at flight levels:

dH*n /dt ≈ 1 - 4 µSv/h at 12 km

Ref.: J.F. Valdéz-Galicia et al., NIMA 535 (2004) 656-664

1 m2 scintillator

anticoin. shield

prop. counters

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Classification of Neutrons

de Broglie wavelength: l = h / p

neutrons

Energy range covered in this lecture

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Secondary Neutrons

Secondary Charged Particles

General Detection Principles

• Neutron detectors do not detect neutrons but products

of neutron interactions!

• Almost all detector types can be made neutron sensitive:

– external converter (radiator)

– converter = detector

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The Neutron Detection Process

• Detection of a neutron is a sequential process:

1. Interaction of the incident neutron: Neutron transport

2. Transport of secondary particles to or within sensing elements:

Hadron, ion, photon transport

3. Primary ionization by secondary particles

4. Conversion to optical photons, gas amplification:

Transport of electrons and optical photons

5. Conversion to electrical signal S

• These steps are described by transfer functions Ti (si-1,si)

• Convolution of the Ti’s: Response function R(S,E)

EEESRSN ES d)(),()(

… How to solve this integral equation?

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General Detection Principles

Basic requirements for neutron detection:

• Slow neutrons: high Q-values, no resonances!

• Fast and high-energy neutrons: large smooth cross sections!

Basic types of neutron detectors:

• Neutron counters

– Signal does not depend on neutron energy

– Typical for detection of thermal neutrons

• Neutron spectrometers

– Signal somehow related to neutron energy

– Inversion procedures are used to infer the

neutron energy distribution

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Interaction of Neutrons with Matter

Neutrons can only be detected after conversion to charged particles

or photons:

• Elastic scattering: AX(n,n)AX recoil nucl. AXz+

• Inelastic scattering: AX(n,n'g)AX recoil nucl. AXz+, e-

• Radiative capture: AX(n,g)A+1Y e-

• Neutron emission: AX(n,2n)A-1Y radioact. daughter

• Charged-particle emission (lcp = p, d, t ,h, a):

AX(n,lcp)A'Y lcp, recoil nucl. A'Yz+

• Fission: n+AX A1X1+A2X2 + nn fission fragments

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Cross Sections Relevant for Neutron Detection

• List of reactions relevant for neutron detection:

Cross section standards + dosimetry standards!

• some additional reactions: 187Au(nth,g), 155,157Gd(nth,g), 209Bi(n,f),

…

209Bi(n,f)

155,157Gd(n,g) 197Au(n,g)

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Kinematics of Nuclear Reactions: A(a,b)B

Kinematical properties of two-particle reactions

relevant for neutron detectors:

• Strict correlation between ejectiles

this is important for tagging neutrons

• Energy EB of recoil nucleus

• Center-of-mass laboratory system

Energy distribution of recoils

this is ‘employed’ in recoil detectors

cm

B

cm

b pp

labB

2

20B cos)(

4

aA

AEE

acm

2

aB d

d

4

)(1

d

dE

A

aA

EE

N

)2/cos(cos cmB

labB

B

b

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Thermal and Slow Neutrons

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Two-Particle Reactions with high Q-Value

• Cross section: (E) = 0 (v0/v), v0 = 2200 m/s

0: Westcott cross section

• Reaction rate indep. of neutron spectrum nE:

nvEvnv

vR E 00

00 d

• 10B(n,a0)7Li: Q0 = 2.792 MeV

10B(n,a1g)7Li: Q1 = 2.310 MeV

• 6Li(n,t)4He: Q = 4.78 MeV

• 3He(n,p)3H: Q = 0.764 MeV

• 235U(n,fiss): Q ≈ 200 MeV

Slow neutrons: En << Q

QEEpp 2121 ,

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BF3 and 3He Proportional Counters

• Cylindrical and spherical shapes

Large variety of sizes: l < 1 m

and pressures: p < 1 bar (BF3), 10 bar (3He)

• Counters must be calibrated: – 3He and BF3 pressure ?

– 10B enrichment ?

– Electrical field ?

– Wall effects ?

• n/g discrimination using a pulse-height threshold

• BF3: aging at high dose rates

air transport prohibited: HF formation!

• 3He: more efficient than BF3because of larger p

low Q-value makes n/g sep. difficult

• 3He is scarce nowadays replacements urgently needed!

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3He and BF3 Pulse-Height Spectra

• Wall effect: incomplete energy deposition by one ejectile:

E1 < Edep < E1 + E2

• Significant dead times: tDT = 1-10 µs

• Photon background suppressed by pulse-height threshold

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Fast Neutrons: Moderating Detectors

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Ref.: G. Knoll, Radiation detection

and measurement, 3rd ed., p. 539

Flat Response Detectors: General Principles

• Fast neutron cross sections are small!

Cover a thermal detector with

a hydrogenous moderator

• Response depends on:

– scattering cross section

– moderator size

– neutron energy

Reliable calculation with

transport codes possible

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Scattering Cross Section for Hydrocarbons

NB: np scattering dominates for En < 20 MeV

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The Long Counter

• Design from the 1950-60s: Hanson, De Pangher, McTaggart

• Design principles

– Thermal shield for

directional response

– Grooves for deeper penetration

of low-energy neutrons

– High sensitivty

• Large device: l = 44 cm, 38 cm

• Flat response:

En = 0.01 - 10 MeV

dR/R ≈ 10%

• Effective centre x0(E):

• Sensitive to room-return but very stable ideal monitor

20 )(

1

xxN

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Monte Carlo Modelling of Long Counters

• Annular moderator and borated shield protect the inner

moderator from neutrons entering from the sides

• Higher energy neutrons penetrate deeper into the moderator

En = 5 MeV

Flat-field irradiation form the right hand side Only neutron ‘tracks’ contributing to the response of the thermal detector are shown!

En = 144 keV

n

Ref.: N. Roberts et al., NPL Report DQL RN004

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Long Counter: Response and Effective Center

• Calibration with radionuclide sources: link to activity standard!

• Overall uncertainty (NPL): uR/R = 0.014, ux0 = 0.63 cm

• Useful energy range: 1 keV – 15 MeV (de Pangher LC)

(w/o carbon resonances)

• Designs with ext. energy range and/or higher sensitivity available

Ref.: N. Roberts et al., NPL Report DQL RN004

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Long Counters for Beta-Delayed Fission Neutrons

• ß-del. fission neutrons: t1/2 ≈ 0.1 – 100 s

• PE moderator with 3He counters

– Fissionable sample in central channel

– Neutron detection eff. e > 10%

– Irradiation sequence:

beam on – beam off and counting

– Precursors kept in equilibrium

Qb

Sn

beam on beam on

beam off

Ref.: X. Ledoux et al., ERINDA workshop 2010

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Fast Neutrons: Recoil Detectors

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Recoil Detectors: General Principles

Recoil detectors are the working horses of neutron metrology

• Based on elastic scattering: Q = 0 MeV

• Most important reaction: 1H(n,n)1H

• Differential response determined by (d/dcm)

• Interference from other constituents and detector properties

• Two approaches for detection of elastic recoils

– Detector = target: full angular distribution

– Separate radiator: only backward angles

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np Scattering: Overview

• Total np cross section: tot ≈ np

• Relative measurements at LANSCE: 5 - 500 MeV

Abfalterer et al. (2001), uncertainty < 1%

• Differential np cross section: (d /d)

relative angular distributions, normalization to tot

– Analytical fit to exp. data: Gammel formula (1960)

– Phase-shift analysis: Hopkins-Breit (1970) ENDF/B-V

– R-matrix analysis: Dodder-Hale (1991) ENDF/B-VI

Dodder-Hale (2006) ENDF/B-VII

• Important for metrology:

Backward scattering (d /d)CM(180°)

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Differential Neutron-Proton Scattering Cross Section

• only minor changes to el

uncertainty 0.3% - 0.5% (E < 20 MeV)

• del / dcm) isotropic

up to about 3 MeV

• 2 % changes for (d /d)

from ENDF/B-V to B-VI

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np Scattering above 20 MeV

• recommend phase shift analysis: VL40

• not many new np data: TSL, IUCF, PSI

• no uncertainties given: ‘about 5%’

n-p backward scattering

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Recoil Detectors: Proportional Counters

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Recoil Proportional Counters

• Strong quenching for low-energy recoil in organic scintillators:

L/E = 0.08 for 100 keV p in BC501A

• Ranges of recoil particles in solids become very small:

R = 1.4 µm for 100 keV p in PE

Gaseous detectors for detection of low-energy recoils

• Complications:

– design more complicated (el. field, surface treatment, cleanliness)

– need for high-vacuum and gas filling systems

– wall effects important

– large non-constant rise-time not well-suited for TOF

– Interference from photons and 12C recoils

• Pioneering work of E.F. Bennet et al. from the 1950’s - 1070’s

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The PTB Proportional Counter P2

• total volume: ≈ 1.6 dm3

• active volume: 55.5 mm, l = 193.3 mm

• el. field: defined by 4 mm field tubes at ground potential

• anode: 100 µm gold-plated tungsten wire (selected)

• counting gas: H2/CH4 (3.5 vol.%), C3H8

• energy range: 20 keV – 2 MeV

Ref.: T.H.R. Skyrme et al., Rev. Sci. Instr. 23 (1952) 204

n

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MC modeling required to describe finite-size and gas-related effects

• incomplete energy deposition • energy-dependent W-value • carbon recoils included • shape of the sensitive volume

Modelling of the RPPC Response to Neutrons

1.9 mm

Stagnation point

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Recoil Detectors: Scintillation Detectors

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The Physics of Organic Scintillators

Unitary scintillators:

• Benzene ring: delocalized p orbitals

• Singlett (1X, 1X*, 1X**) and triplet (3X*, 3X**)

states

Principal physical processes:

• Excitation by electron impact, ion-ion

recombination, …

• Non-rad. efficient internal degradation 1,3X** → 1,3X*+ phonon

drain via competing channels:

quenching states

• Rad. decay 1X* → 1X:

prompt fluorescence:t = 1 – 80 ns, lfluor > labs

• Rad. transition 3X** → 1X* forbidden

• Coll. deexc. 3X* + 3X* → 1X* + 1X + phonon:

delayed non-exp. fluorescence: t > 300 ns

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Ionization Quenching and Pulse-Shape Discrimination

• Increased ionization density:

– More ion-ion recombination

– Ratio of 1X** and 3X** excitations increases

– Temporal concentration of transient

quenching states (t 100 ps) decreases

Delayed fluorescence less dependent

on dE/dx than prompt fluorescence

• Light yield dL(E)/dx is a non-linear function

Semi-empirical formulas (Birks et al.):

• Scint. decay depends on dE/dx:

pulse-shape discrimination of

particle species (Z, A)

...)d/d1(

d/d

d

d

xEkB

xES

x

L

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Bi-alkali photocathode

aka: BC400, NE102-A

Liquid and Plastic Scintillators

• Typical unitary scintillators:

– Stilbene (1,2-Diphenylethene C14H12)

– Anthracene (C14H10): ‘gold’ standard for

scint. efficiency

• Binary or ternary scintillators

– Solvent X: Benzene (C6H6),

n-Methylbenzene (C6H6-n(CH3)n),

Styrene [C6H5·C2H3]n , ….

– Solutes Yi: PPO, POPOP, bis-MSB, ….

– Lower excitation energy: EY* < EX*

• Prim. processes as in unitary

scintillators → 1X*, 3X*

• Energy transfer to solutes 1X* + 1Y → 1X + 1Y* (rad., non-rad.) 3X* + 1Y → 1X + 3Y* (non-rad.)

→ rise, decay times

• Secondary solute: wavelength shifter

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Light Output of Organic Scintillators

• Light Output calculated from

Birks‘ parameterization:

– Weak ionization:

– Dense ionization:

• Electrons: Le(E) = Se(E – E0)

Se := 1 MeV-1, E0 ≈ 5 keV for BC501A

• Also higher-order formulas are approximations!

• Note:

– Experimental data include instrumental effects!

– Electrons produce Cherenkov radiation for b > n-1:

Ee > 170 keV in BC501A

E

Edx

dE

dxEkB

xESEL

0

1

d)/d(1

)d/d()(

)()( ERkB

SEL

ESEL )(

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Properties of Organic Scintillators

plastic scint. liquid scint.

1. Hydrogen / carbon ratio ≈ 1.1 ≈ 0, 1.2 – 2.0

2. Scintillation efficiency 55 – 65 % 40 – 80 % anthracene

3. Scintillation spectrum lmax 370 – 490 nm ≈ 425 nm

4. Transparency 1 - 4 m

5. Decay times 1.4 – 3 ns, 230 ns 2 – 4 ns

6. Pulse-shape discrimination (yes) yes

7. Doping for thermal sensitivity yes yes

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Components of a Liquid Scintillation Detector

• Scintillator cell + expansion volume

• Light guide (reflective coating)

• PMT with µ-metal shield

• High voltage supply

– resistor chain + decoupling capacitors

– transistorized low-power dynode supplies

• Gain stabilization

– Count rate drifts

– Temperature drifts

LED or laser-based

systems

Prototype FD detector:

12"2" NE213

Scint. cell

Coated light guide

PMT (Valvo)

Resistor chain

Expan.

volume

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Response of Organic Scintillation Detectors

• Elastic n-p scattering cross section

dominates: dN/dL = const for L < En

Modification of the rectangular shape:

• Non-linear light output: L E3/2

dN/dL = (dN/dE)·(dL/dE)-1 L-1/3

• n-12C scattering: DEn< 0.28·En

• Multiple n-p scattering: SL(Ep,i) < En

• Finite pulse-height resolution

• 12C(n,x) reaction:

Q value for 12C(n,n',g): 4.4 MeV

Simulated using Monte Carlo techniques!

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12C(n,x) Reactions for En < 20 MeV

En < 15 MeV: only scattering and alpha emission!

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Break-Up Reactions

En < 20 MeV: 12C(n,n'3a)

• Four-body break-up with

several channels

• Investigated using nuclear

emulsions and liquid

scintillation detectors

• Still insufficient data for MC

codes

– NRESP7

– Geant4 (data base from CIEMAT)

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Example: 2"x2" BC501A Detector at En = 15 MeV

• Simulation using NRESP7, similar results with SCINFUL

• Generally good agreement for En < 16 MeV, but

description of a emission channels still problematic!

• Partial spectra can be sorted by first interactions

• Pulse-height spectra used to determine 12C(n,a)9Be cross section! Ref.: H.J. Brede et al., NSE 107 (1991) 22

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Response for En > 20 MeV

Response dominated by n-12C interaction:

• Strong contributions from break-up reactions:

correlation of charged particles from individual interactions!

• Data libraries (ENDF, JEFF) have only emission tables:

general-purpose MC codes are not adequate: MCNPX, Geant4

0 5 10 15 20 25 30 350

500

1000

1500

2000

2500

En = 45 MeV

exp.

SCINFUL

(dN

/dL)

/ a

rb.

un

its

L / MeV

n-p scattering

12C(n,d)

12C(n,p)

12

C(n,px)

+12

C(n,dx)

+12

C(n,ax)

+ ....

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Pulse-Shape Discrimination (PSD)

• PSD properties depend on:

– Neutron energy

– Detector size

– Multiple scattering

– Scintillator composition (oxygen, impurities)

• Discrimination of neutron and

photon-induced events

– Ambient photons: background reduction!

– Delayed gammas

– Suppression of response induced by 12C(n,n'g)

• PSD yields Z/A information: particle identification

• Techniques:

Analogue RC(CR)2 shaping → zero crossing

– Analogue or digital integration: Qshort vs. Qlong

– Fit of the waveform

• PSD has limited dynamic range: difficult for L < 500 keV

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PSD Examples

• Many older MC codes do not include the 12C(n,n'g) channel.

Photon-induced events must be excluded

• Problem: Separation of photon-induced and proton escape events

L = 100 keV

1"x1" BC501A

En = 3.5 MeV

2"x4" BC501A

En = 66 MeV

a t d

p

e escaping p

Pulse Height L

PSD

PSD

Pulse Height L

PSD: analogue RC(CR)2 technique

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Pulse-Height Resolution

• Pulse-height resolution depends on

– Light collection: A

– Photoelectron statistics: B

– Electronic noise: C

• Elongated cells for high-energy

neutrons:

– Long tracks

– A depends on neutron energy En

• Optimisation important for

spectrometry

– Small cells (diameter ≈ depth)

– Partially coated light guides

• Unfolding: DEn/En ≈ 0.2·DL/L

222 LCLBAL

L

D

2“2“ BC501A cell

2“4“ BC501A cell

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Experimental Characterization of Detectors

• Experimental data to be

measured for each detector:

– Pulse-height resolution

– Light output function

– Response matrix (normalized to n-p

scattering)

• Suitable neutron beams:

– Monoenergetic: time consuming!

– ‘White’ (Be+p, C+p, D+d):

En selection via TOF

– TCAP: ‘absolute’ measurements

• Normalization in the n-p part:

response matrix: (dR/dL )(En)

• This works up to 60-70 MeV

TOF

Pu

lse

he

igh

t

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Results

Investigation of a set of NE213/BC501A detectors

• LO: up to 10% deviation from ref. data

• R: up to 4% rescaling of calculated response functions

Re-normalization factors for NRESP7 response functions for monoenergetic neutrons

Ref.: D. Schmidt et al., NIMA476 (2002) 186-189

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Recent Developments: PSD with Plastic Scintillators

• Liquid and plastic scintillators: binary or ternary systems

• Properties determined by primary solute system:

PSD: 3Y* + 3Y* → 1Y* + 1Y

– Liquid scintillator: strong molecular diffusion of 3Y*

– Plastic scintillator: long-range dipole – dipole interactions required

this process is only effective at higher Y concentrations

Increased concentration of primary solvent could improve PSD

Ref.: N. Zaitseva et al., NIMA668 (2012) 88.93

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Commercial Plastic Scintillator with PSD: EJ299-33

• PSD properties as good as for liquid or crystal scintillator

• Secondary solute improves QE

• Practical advantages: non-toxic, non-flammable, no container,

all shapes, sizes 15 cm

n

n,FOM

dd g

g

S

Ref.: N. Zaitseva et al., NIMA668 (2012) 88.93

aka BC501A

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Boron-Loaded Liquid Scintillators

• Main disadvantage of organic scintillators: strong quenching

Boron doping increases sensitivity to low energy neutrons: 10B(n,a0)

7Li + 10B(n,a1)7Li*

– Up to 4.5 % (wt.) B loaded as B(OCH3)3

– Commercially available, e.g.: EJ309B, BC523A

– Dopant influences light output and PSD!

• Same technique applicable for 6Li and 157Gd

• 10B-loaded plastic scintillators available as well: BC454

Ref.: J. Iwanowska et al., JINST 7 (2012) C4004 (FNDA2011)

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Ce3+ activated lithium silicate glass

• SiO2: 55-75%, MgO: 4-25%, Al2O3: 9-20%,

Ce2O3: 4-5%, Li2O: 6-21%

• Depleted ( 0.01%) and enriched ( 99.9%) in 6Li

Low-background material: NE912 / NE913

• Low light yield: 5% of NaI for p, He

15-25% of NaI for electrons

• Poor PSD properties

• Limited n/g discrimination by PH threshold

• Non-linear light output: La(v) Lp(v)

• Main application: TOF spectrometry

Inorganic Scintillators: 6LiGlass

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Novel Inorganic Materials: CLYC

CLYC: Cs2LiYCl6:Ce

• High light yield: 40 - 60 % NaI for p, He

60 – 90 % NaI for electrons

• Good PH resolution g and n detector

• Neutron detection: 6Li(n,a), 35Cl(n,p)

• Excellent PSD properties

• Excellent time resolution: Dt < 800 ps

• Cracky crystals

• Small sizes: < 2"

• Expensive material

Therm. n 1.3 MeV

Ref.: N. D‘Olympia et al., NIMA 714(2013) 121-127

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Additional Material

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Spherical 3He Counters

Centronics SP9 Counter: - almost isotropic response - 3He pressure range: 0.2 bar – 2 bar - working horse for thermal neutron measurements

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TOF Long Counters: Black Detector

• Moderation time in a long counter: several 10 µs

not suited for time-of-flight (TOF)

• Black detector:

- Moderator: liquid scintillator

- Efficiency 0.95 ± 0.05

for En = 0.5 – 10 MeV

- Time response determined by Lp(E)

- TOF resolution 4 ns (tail!)

Ref.: W.P. Pönitz, NIM 109 (1973) 413-420

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Long Counters for Beta-Delayed Neutrons

• ß-del. neutrons in r-process nucleosynthesis:

path back to stability: A → A-1

add. neutron source: Pn

• BELEN-30 detector:

1 m3 PE moderator

30 3He counters in two ‘crowns’

Precursors implanted in Si-strip detectors

Recording of b- and n-events

Exp. verification of the MCNPX model: 252Cf(s.f), 13C(p,n), 13C(a,n), 51V(p,n) at PIAF

Ref. :M.B. Gómez-Hornillos et al., JPConf 312 (2012) 052008

to be verified !

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Photon/Neutron Discrimination

Rise-time measurement:

fast filter amp. : tdiff = 50 ns, tint = 500 ns

• start: LE-disc. close to noise • stop: CF-disc. (f = 0.4)

shaping amp.: ts = 2 µs

Pulse height

Ris

etim

e

965/35 hPa D2/CD4

Recoil tracks more localized than electron tracks different drift times for sec. electrons

NB: Analogue technique to be replaced by waveform digitizers!

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Optimization of the Pulse-Height Resolution

Light transport: - refraction and reflection

- total reflection

- diffuse reflection on TiO2 coating

radius

ligh

t co

llect

ion

Ref. H. Schölermann et al. NIM 169 (1980) 25-31

Many Monte Carlo codes available Optical properties treated as free

parameters No quantiative predictive power

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‘Unconventional’ Liquid Scintillators: LAB for SNO+

• SNO+: 0n2b decay of 130Te, …

• Scintillator: 780 t (LAB + 2 g/l PPO)

• Quenching data required for background model

15 mg/l bis-MSB (wavelength shifter) Poor PSD properties proton quenching: kB = 0.0093 Design value kB = 0.0073

Important for the background model

Ref.: B. v. Krosigk et al., EPJC (2013) 73:2390